[llvm] [RFC][BPF] Support Jump Table (PR #133856)

[Anton Protopopov via llvm-commits]

aspsk wrote:

> You find a gotox insn with target r2, then you need to go back and find 'r2 = *(u64 *)(r2 + 0x0)' and then 'r2 += r3' and then 'r2 = 0x140 ll'. The above code pattern is gernated by llvm and should be generally true for jump table implementation. And you will be certain that the table for this particular gotox will be in offset 0x140 of .rodata section. The size of the table is already calculated based on the previous mechanism by scanning all .rodata relocations in .text and decap sections.

I am looking into how to automate this properly (I have a _really hacky_ PoC test working with this version of llvm and my custom test). It looks simpler with explicit jump tables (when I take an address of a label and store in an array), because then I can just push values to a custom section.

Will post updates here.

https://github.com/llvm/llvm-project/pull/133856